1. Field of the Invention
This invention relates generally to internal combustion engine spark ignition systems, and more specifically to a pencil ignition coil assembly module that mounts directly on an engine in direct electric connection with an engine-mounted spark plug.
2. Background Information
Known internal combustion engines comprise cylinder blocks containing individual cylinders that are closed at one end by an engine cylinder head that is attached to the engine block. In a spark-ignition engine, the cylinder head contains threaded spark plugs holes, each of which is open to a respective cylinder. A respective spark plug is threaded into the respective hole to close the respective hole. External to the respective cylinder, each spark plug comprises a central electric terminal that is available for electric connection with a mating terminal of a secondary of the spark-ignition system.
Known spark ignition systems comprise what are sometimes called coil-on-plug type ignition coil assemblies. Each such assembly comprises both a wound primary coil and a wound secondary coil. At the proper time in the engine operating cycle for firing a particular spark plug, electric current flowing through the primary of the respective ignition coil assembly is abruptly interrupted to induce a voltage in the secondary coil sufficiently high to create a spark across gapped electrodes of the spark plug that are disposed within combustion chamber space of the respective engine cylinder, igniting a combustible fuel-air mixture to power the engine.
Examples of coil-on-plug type ignition coils are found in U.S. Pat. Nos. 4,514,712; 5,128,646; 5,590,637; and 5,870,012; as well as in U.K. Patent Application GB 2,199,193A. A common characteristic of such coils is that the primary and secondary coils are disposed one within the other, concentric with a common axis that is coincident with the axis of the spark plug electrode. The coils may be bobbin-mounted and encapsulated. Each of U.S. Pat. Nos. 5,128,646; 5,870,012; and U.K. Patent Application GB 2,199,193A shows the wound primary coil disposed interiorly of the secondary winding while the other two show the reverse. Various arrangements for providing electric circuit continuity of the secondary coil to the spark plug terminal are shown.
In certain engines, the threaded spark plug mounting hole may be at the bottom of a bore, or well, that extends inward from an outer surface of a cylinder head. For any of various reasons, such bores may be relatively long and narrow. It is for such bores that pencil ignition coil assembly modules are especially suited. U.S. Pat. No. 5,870,012 shows an example of a relatively long and narrow ignition coil assembly that is inserted for a majority of its length within a bore leading to a spark plug mounting hole. At its upper end, that ignition coil assembly has a connector disposed external to the bore and containing electric terminals providing for connection of the primary coil with the ignition system. An advantage of a pencil-type ignition coil is that when it is installed on an engine, the wiring that runs to it from a signal source need carry only primary coil current, because the entire secondary coil is contained within the module and is for the most part sheltered within the bore.
For proper ignition system performance, primary and secondary coils of an ignition coil assembly must be sized to reliably deliver a secondary voltage sufficiently large to spark the plug. The primary and secondary coils are typically encased in respective encapsulations which must possess physical characteristics suitable for providing protection both for the harsh ambient environment where the ignition coil is located and for the voltages generated. Because of dimensional constraints that may be imposed on a pencil-type ignition coil by the design of an engine, it is believed that an ignition coil possessing an ability to achieve specified performance criteria within confined space would be valuable to an engine manufacturer.